The present invention relates to a device for converting alternating voltage into direct voltage and conversely, which comprises a series connection of at least four units each consisting of a semiconductor element of turn-off type and a first diode connected in anti-parallel therewith, said series connection being arranged between two poles, a positive one and a negative one, of a direct voltage side of the device, an alternating voltage phase line connected to a first mid point, which is called phase output, of the series connection between two units while dividing the series connection into two parts, means adapted to provide a mid point between the two poles on said direct voltage side and put these poles on the same voltage but with opposite signs with respect to the mid point of the direct voltage side, a second mid point of each said part of the series connection being through a second diode with the conducting direction with respect to the phase output opposite to the conducting direction of the first diode in the unit arranged between this second mid point and the phase output connected to the mid point of the direct voltage side and an apparatus for controlling the semiconductor elements of the units to generate a train of pulses with determined amplitudes according to a pulse width modulation pattern on the phase output of the device by alternatingly connecting the alternating voltage phase line to at least the mid point, the plus pole and the minus pole of the direct voltage side.
Such devices may be used in all kinds of situations, in which direct voltage is to be converted into alternating voltage or conversely, in which examples of such uses are in stations of HVDC-plants (high voltage direct current), in which direct voltage normally is converted into a three-phase alternating voltage or conversely or in so called back-to-back-stations in which alternating voltage is firstly converted into direct voltage and this is then converted into alternating voltage, as well as in SVCs (Static Var Compensator), in which the direct voltage side consists of one or more capacitors hanging freely.
Such converter devices already known have a number of drawbacks, when these are used for transmitting high powers, and the present invention aims at a converter device being well suited to transmit high powers, although the invention is not restricted to this field of use, since a converter device of this type may very well find other field of uses. However, the case of transmitting high powers will for this reason hereinafter be discussed for eliminating but not in any way restricting the invention.
The device defined in the introduction is a so called multi-level converter, since it may deliver at least three different phase potentials on said phase output. Different types of such multi-level converters of this voltage stiff so called VSC-type (Voltage Source Converter) for high power applications have been described in the IEEE-article IEEE Trans. on Ind. Appln. Vol 32, no 3, 1996, pages 509-517. Three different types of multi-level converters are described therein, namely multi-level converters with clamping diodes, multi-level converters based upon flying capacitors and multi-level converters based upon cascaded converters. Only the two first ones are suitable for transmitting active power, such as for example in HVDC- and back-to-back-applications. The greatest problem of multi-level converters having clamping diodes is that the diode cost will be very high when the number of levels increases, so that for example in the case of five levels the number of clamping diodes increases so that there is a need of more clamping diodes then said semiconductor elements of turn-off type. The converter devices with flying capacitors require for sure no clamping diodes, but they require instead a large number of capacitors, and the capacitor size increases with a comparatively large factor when the number of levels is increased, in which this factor is for example five to six when it is changed from three to five levels. Accordingly, this solution is also very costly.
The object of the present invention is to provide a converter device of the type defined in the introduction, which is well suited for high voltage and high power applications and in which the drawbacks mentioned above of such devices already known are reduced to a large extend, primarily at an increased number of levels of the converter.
This object is according the invention obtained by connecting a semiconductor element of turn-off type in anti-parallel with each of said second diodes in a device of the type mentioned in the introduction, and that the apparatus is adapted to control the semiconductor elements of the units between the two second mid points to be turned on and turned off with a pulse width modulation frequency of at least one order of magnitude higher than the fundamental frequency of the alternating voltage of said alternating voltage phase line and to control the semiconductor elements connected in anti-parallel with said second diodes and in the units between the respective second mid point and the respective pole to be turned on and turned off with a frequency being substantially lower than said pulse width modulation frequency and within or close to the frequency range one or a couple of times said fundamental frequency.
By arranging a semiconductor element of turn-off type in this way in anti-parallel with said second diodes it is possible to also control the connection of the mid point of the direct voltage side to the second mid point, and it gets possible to obtain a desired pulse width modulation pattern at the connection of the phase line to the phase output by turning these semiconductor elements on and off as well as those arranged between said second mid point and the respective pole with a comparatively low frequency in the order of the fundamental frequency of the alternating voltage of the alternating voltage phase line, whereas the semiconductor elements of the xe2x80x9cinnerxe2x80x9d units are turned on and off with a significantly higher frequency, more exactly the so called real pulse width modulation frequency. The frequency through which the semiconductor elements first mentioned are switched may for example be 50 or 60 Hz, while the pulse width modulation frequency is typically 1-2 kHz. This means that totally different, more exactly lower, demands are made upon the semiconductor elements first mentioned, which have not to be turned on and turned off with any high frequency, which means that for this semiconductor elements having a considerable better ability to hold high voltages may be used, since such high voltage semiconductor elements cannot take high frequencies without unacceptably high switching losses. Voltages in the order of 10-400 kV are normally handled in devices of this type, and this requires then a series connection of a higher number of semiconductor elements within each said unit for a series connection of a higher number of said units so as to distribute the voltage these have to hold in the blocking state among a high number of such semiconductor elements. Thus, in the present case it will be possible to use a lower number of semiconductor elements connected in series between said second mid point and the respective pole, since these may be of high voltage type, for example hold 4-6 kV instead of 2-3 kV, which means a considerable saving of costs and simplifies the control of the device. Semiconductor elements with a smaller component area may alternatively be used, which have a higher thermal resistance, but which are available to a low cost, may be used for these semiconductor elements switched comparatively seldom. The same condition is valid for the semiconductor elements connected in anti-parallel with the second diodes and switched with a low switching frequency.
According to a preferred embodiment of the invention the apparatus is adapted to control the semiconductor elements connected in anti-parallel with said second diodes and in the units between the respective second mid point and the respective pole with a frequency coinciding with said fundamental frequency in absence of voltage harmonics in the alternating voltage phase line. A large difference in frequency between the control of these semiconductor elements and the other semiconductor elements is obtained by this and the advantages mentioned above of the invention with respect to the devices already known will by this be very remarkable. The apparatus is advantageously adapted, when said voltage harmonics occur to optionally carry out one or several additional switchings of the semiconductor elements connected in anti-parallel with said second diodes and those in the units between the respective second mid point and the respective pole within a fundamental frequency period, in which a switching is defined as comprising a turn-off and turn-on. It may in this way be compensated for such instabilities, in which the frequency in question may during a very short period of time become for example three times said fundamental frequency, but it is still considerably lower than the pulse width modulation frequency.
According to another preferred embodiment of the invention the apparatus is adapted to control the semiconductor elements of the units and the semiconductor elements connected in anti-parallel with said second diodes to alternatingly connect the alternating voltage phase line to an odd number of different levels, in which one of them is the mid point of the direct voltage side and just as many are positive as negative, in which said number is n, which is at least five, that at least (nxe2x88x921)/2 of said units are connected in series between the second and the first mid point, that it comprises (nxe2x88x923)/2 so called flying capacitors and that each said flying capacitor is connected with one pole thereof to a mid point of said series connection, which is located between the phase output and the second mid point on the opposite side of the phase output with respect to the connection mid point belonging to the opposite pole thereof and has at least one unit between itself and the second mid point and another unit between itself and another capacitor connection or the phase output. Such a multi-level converter device with a higher number of levels than another converter device, which has for example three levels, results in a better adaptation of the pulse width modulation pattern to the sinus wave desired to be obtained downstream of an inductor or transformer arranged in said alternating voltage phase line, so that the harmonics generated during the conversion are reduced or the size of these inductors and/or filters for extinguishing such harmonics may be reduced, lower voltage differentials may be obtained for said inductors or transformers, so that stresses thereon may be reduced and these may be made less costly, and lower switching losses may be obtained. Such converter devices with a higher number of levels and the advantages associated therewith may according to this advantageous embodiment of the invention be obtained in a simple way and to a low cost. The advantages of the lower frequency of the semiconductor elements arranged between the respective second mid point and the respective pole as well as the semiconductor elements connected in anti-parallel with said second diodes have been discussed above. In addition thereto, this way to arrange a flying capacitor is very advantageous with respect to the arrangement of flying capacitors of the second type mentioned above of converter devices based upon flying capacitors, since a flying capacitor (s) are connected in such a way that they across the poles thereof will have a considerably lower voltage than in the case of the flying capacitors of the devices already known, more exactly the voltage across the flying capacitor with the highest voltage thereacross is in the invention preferably not more than half the voltage across the entire series connection, which is of a great importance, since the power to be handled by a capacitor is proportional to the square of the voltage, so that the present invention enables a use of flying capacitors while avoiding the large number of clamping diodes which would be necessary in the case of a converter device of the type first mentioned, and the drawbacks of the second type of converter devices based upon flying capacitors has with respect to requirements of very large capacitors for a large number of levels of the converter device are nevertheless avoided. A large advantage of a converter device according to this embodiment of the invention is accordingly that it is possible to get a five-level-converter to a comparatively low additional cost with respect to a three-level-converter, and it will be easy to modify a three-level-converter.
According to a preferred embodiment of the invention n is 5 and said units are adapted to give the flying capacitor a voltage across the poles thereof substantially equal to U/4n, in which U is the voltage between the two poles of the direct voltage side. The voltage of the flying capacitor may in this way be kept low and the size and the cost thereof may be kept at a low level.
According to another preferred embodiment of the invention the apparatus is adapted to control said units, when one pole of said flying capacitors is connected to said phase output so that the phase current passes said capacitor, to make this connection in one of two ways, which gives substantially the same phase potential on the phase outlet depending upon the instantaneous real level of the voltage between the poles of the capacitor, so that the capacitor is upon said connection charged for a voltage level thereof lower than desired and discharged for a voltage level thereof higher than desired. This process is possible thanks to the fact that there are two possible states giving almost the same potential on the phase output, in which one state may be used for charging the capacitor and the other for discharging the capacitor for a given direction of the phase current. This process means that the capacitance value of the capacitor may be kept at a minimum, with a time constant for the charging and the discharging, respectively, which is a suitable factor higher than the period of time during which the capacitor is normally switched in each of the positions for a given switching frequency.
Further advantages as well as advantageous features of the invention appear from the following description and the other dependent claims.